Functional & Performance Testing
VO2 max, continuous glucose monitoring, heart rate variability, gut microbiome testing, and metabolomics — the tests that measure how your body actually performs.
Key Takeaways
- →VO2 max is one of the strongest independent predictors of all-cause mortality — stronger than smoking, diabetes, or hypertension.
- →Continuous glucose monitoring (CGM) reveals real-time glycemic variability invisible to standard blood tests.
- →Heart rate variability (HRV) reflects autonomic nervous system health and can track recovery, stress, and fitness trends.
- →Gut microbiome testing is commercially available but clinically actionable insights remain limited for most healthy adults.
Traditional lab tests measure what is in your blood. Functional tests measure how your body actually performs under real-world conditions. How efficiently does your body use oxygen? How does your glucose respond to specific foods? How well is your autonomic nervous system regulating your body? These tests provide a dynamic picture of health that static blood draws cannot capture.
VO2 Max Testing
VO2 max — maximal oxygen consumption — measures the peak rate at which your body can utilize oxygen during intense exercise. It is expressed in mL/kg/min and reflects integrated cardiopulmonary and muscular fitness. A landmark 2018 study in JAMA Network Open analyzed over 122,000 patients and found that cardiorespiratory fitness (as measured by estimated VO2 max) was inversely associated with all-cause mortality, with no upper limit of benefit. Extremely fit individuals had the lowest mortality risk.
VO2 max and longevity
Dr. Peter Attia has emphasized VO2 max as perhaps the most powerful marker for longevity. Moving from the bottom 25th percentile to above the 50th percentile in cardiorespiratory fitness is associated with a roughly 50% reduction in all-cause mortality risk — a larger effect than achieved by most medications.
Clinical VO2 max tests use a graded exercise protocol on a treadmill or cycle ergometer with a metabolic cart analyzing expired gases. Consumer alternatives include estimated VO2 max from devices like the Apple Watch, Garmin, and WHOOP, though these are less precise than direct measurement.
Continuous Glucose Monitoring (CGM)
A CGM is a small sensor (typically worn on the upper arm) that measures interstitial glucose every 1–5 minutes for 10–14 days. While designed for diabetes management, CGM has gained popularity among non-diabetic individuals for revealing postprandial glucose spikes, overnight glucose patterns, and the glycemic impact of specific foods, exercise, stress, and sleep.
- •What CGM reveals — Foods that spike your glucose, the effect of exercise timing on glucose control, overnight glucose stability, and dawn phenomenon patterns.
- •Limitations — Non-diabetic glucose variability is generally modest, and clinical evidence for CGM improving outcomes in non-diabetic individuals is still limited. The American Diabetes Association does not currently recommend CGM for general wellness use.
- •Popular options — Dexcom G7, Abbott FreeStyle Libre 3, and consumer programs from Levels, Nutrisense, and Signos.
Heart Rate Variability (HRV)
HRV measures the variation in time between consecutive heartbeats, reflecting the balance between sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) nervous system activity. Higher HRV generally indicates better autonomic flexibility, cardiovascular fitness, and stress resilience. HRV declines with age but can be improved through exercise, sleep optimization, and stress management.
Gut Microbiome Testing
The gut microbiome — the trillions of bacteria, fungi, and viruses in your digestive tract — plays roles in immunity, metabolism, mood, and inflammation. Companies like Viome, ZOE, and Thorne offer stool-based microbiome sequencing. However, the science of translating microbiome data into actionable dietary recommendations remains early-stage. The American Gastroenterological Association has noted that for most healthy individuals, clinical utility of microbiome testing is limited outside of specific conditions like C. difficile infection.
Manage expectations for microbiome testing
While gut microbiome research is one of the most exciting fields in medicine, current commercial tests provide a snapshot of microbial composition — not a reliable diagnostic or prescriptive tool for most healthy adults. Results can vary significantly between tests and even between samples taken days apart.
Metabolomics
Metabolomics analyzes the complete set of small molecules (metabolites) in blood, urine, or other biofluids. These metabolites are downstream products of gene expression, protein activity, and environmental exposures, providing a real-time snapshot of physiological state. While still primarily a research tool, some companies now offer consumer metabolomic panels that measure hundreds of metabolites to assess mitochondrial function, oxidative stress, nutrient status, and toxic exposures.